Functional Consequences of Delayed Photoreceptor Ablation in Transgenic Mice

Abstract

Abstract: : Purpose: To characterize the functional changes in the retina of a transgenic mouse line (rdta^late) with delayed rod photoreceptor degeneration. Methods: We have previously reported the effects of transgene-induced rod photoreceptor degeneration in rdta mice, which express an attenuated diphtheria toxin gene under the control of a rhodopsin promoter. In a third line of these mice, the phenotype is delayed with rod ablation evident after 42 days of age. Once begun, rod degeneration follows a time course similar to the other two lines and appears to be complete by 60 days of age. This delay in transgene expression in the rdta^late mice allows both the rod and cone photoreceptors to complete normal development before the onset of degeneration. To characterize retinal function in these animals, the full-field light-adapted ERG was recorded from anesthetized mice using a DTL fiber electrode. A xenon arc lamp light source was used to present broad-band light flashes, 10ms in duration, with a 1 sec inter-stimulus interval. Light-adapted mice were positioned in the recording apparatus, and then the room lights were extinguished to begin dark-adaptation. A constant intensity flash was used to elicit the ERG at 1 min intervals for 15 min, and then at 5 min intervals up to 40 min. Thirty responses were averaged at each time point. Normal mice (C57B6/J) were used as age-similar controls. Dark-adaptation functions were constructed by plotting the ERG response as a function of time in the dark. Results: In normal mice, the dark-adaptation curve was described by a two-limbed function, characteristic of the rod-cone transition that occurs during dark-adaptation. Dark-adaptation in rdta^late mice was similar to controls up to about 45 days of age. At that age, a change in the dark-adaptation curves becomes evident with a shallower scotopic limb. By about 70 days, the dark-adaptation curves consist of only a cone limb. After 120 days there is little in the way of an ERG response, suggesting that cone function eventually becomes compromised. Conclusions: These data suggest that, during a window around 50 days of age, the retinal response of rdta^late mice is dominated by cone photoreceptors. The electrophysiology is consistent with the time course of the anatomical degeneration of the rods in rdta^late mice, which is complete around the same age. These data suggest that rdta^late mice may serve as a model to study both the functional consequences of rod dystrophies, and within a certain time frame, the cone pathways in isolation.

Keywords: electroretinography: non-clinical • transgenics/knock-outs • photoreceptors: visual performance